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ECMAScript 5 'Strict mode' support in browsers. What does this mean?

If you are closely following how the web is shaping up lately, you would be thrilled, like me, looking at pace at which things are changing. There are hundreds of things happening on the new HTML5, CSS3, JavaScript front, frequently releasing browser versions and contest for speed, supporting modern standards etc. One such innovations is ECMAScript 5 Strict Mode support in modern browsers. Recently, IE10 platform preview 1 announced 'strict mode' support in Mix 2011 conference . The goal of this article is to explain what 'Strict Mode' support in modern browsers mean and how developers can take advantage of it.


I am aware of JavaScript. But what is ECMAScript 5?

Similar to W3C which standardizes HTML specifications, ECMA is the body which standardizes JavaScript. ECMA-262 is the specification which defines the JavaScript language, interchangeably referred as Jscript/ECMAScript. The JavaScript code which we web developers have been using since a decade is as per the guidelines defined in the Third edition ofECMAScript (ES3 in short) released in December,1999.


ES3 was released in a hurry and it had several loopholes, which means that the future versions have to be built on top of these loopholes to support older pages (I strongly recommend reading grandpa Crockford's book, JavaScript- The Good Parts to know the good and bad parts of the language). ECMAScript 4 had several ambitious, drastic changes in its specs but was never released due to conflicts regarding backward compatibility, fearing a broken web. So,ECMAScript 5 (ES5 in short) was released in December 2009, more as a transitional version of the specification, adding few cool features, without much changes in the syntax (Check the ECMAScript specs archive).


Few new features in ES5:

ECMAScript 5 includes few new APIs, computational methods for things which can already be done in ES3. One of the most interesting features is the newer object model in ES5, using which, newer objects can be created easily and their extensibility can be controlled. Similar to higher programming languages like Java or C#, JavaScript objects now have getters and setters, non-enumerable methods, sealed and frozen objects  etc. ES5 also describes native JSON support, which includes methods like JSON.parse() and JSON.stringify() which remove dependency on external libraries for working with JSON.


Each browser has its specific implementation of ES5, as per its JS engine. ES5 support in IE 9 is different from ES5 support in FF 4.


ES5 Strict mode:

'Strict mode' is an innovative feature of ES5. It is a restrictive variant of ES5 implementation, which follows a slightly different semantics from the normal JavaScript. Someof the deprecated, bad parts of JavaScript in ES3 specification are disabled or thrown as exceptions in 'strict mode'. It is not backward compatible with older browsers and has to be opted-in by the developer.

To enable ES5 in strict mode, just use the string "use strict" as the first line of your JavaScript code. This will make the entire program to run in strict mode.


Alternately, you can place "use strict" in a function, to restrict only to that context.


function isStrictMode(){

return !this;


//returns false, since 'this' refers to global object and '!this' becomes false


function isStrictMode(){   

"use strict";

return !this;


//returns true, since in strict mode, the keyword 'this' does not refer to global object, unlike traditional JS. So here,'this' is null and '!this' becomes true.


I hope to cover detailed demos on this in the following posts.


The good parts of  ES5 Strict Mode:

In JavaScript, if a variable is not explicitly declared, it is implicitly assigned global scope.This may look fun to code in JS, but is a nightmare to maintain in huge projects. Strict mode throws error if variables are assigned values without declaring and hence implicit global variables will not be created. This is a huge win for the language as well as for huge projects.


Few such good changes in strict mode are: Assignment to a read-only property will throw error, object literals having 2 or more properties with same name will throw error, functions having 2 arguments with same name will throw error etc. All these changes will help in making the code more reliable, readable and manageable.


The IE team launched  a 'Strict Mode' demo in their IE Test Drive HTLM5 demos. You can check if your browser supports ES5 Strict mode or not by checking the "Try Strict Mode" demo. Download IE10 platform preview 1 if you haven't and start playing with it!


P.S: Happy to share that Microsoft has recognized my community contributions and awarded me "Microsoft MVP award" for the second consecutive year. In 2010, it was under ASP.NET/jQuery category and now for 2011, it is under "InternetExplorer" category. Thanks to Microsoft for the encouragement Smile

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